Area Forecast Discussion
Issued by NWS Charleston, SC

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FXUS62 KCHS 230240
AFDCHS

Area Forecast Discussion
National Weather Service Charleston SC
1040 PM EDT Fri Sep 22 2017

.SYNOPSIS...
High pressure will prevail through this weekend. The high will
weaken and shift inland as Hurricane Maria tracks northward off
the Southeast U.S. coast through the first half of next week. A
cold front could approach the forecast area late next week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
As of 1035 PM: Conditions across the region have become
generally clear with light to calm winds. A couple of areas that
were within this afternoons rainfall have started to observe
light fog. Based on the recent observation trends, it appears
that the current highlighted area for patchy fog looks good.
Only minor adjustments to sky, temps, and PoPs will be made for
this update.

As of 750 PM: Radar rainfall estimates indicate that a fair
amount of coverage across Charleston, Dorchester, and portions
of Colleton Counties. NAM12 sfc condensation pressure deficits
keep values of 5 mbs or less just inland of the coastal
counties. I will keep a mention of patchy fog over areas of wet
soil and lower pressure deficits.

As of 535 PM: Latest SPC normalized CAPE analysis indicated that
a west to east line of .1 units has moved south across the CHS
metro area. Areas to the south of the line should continue to
see SCHC to CHC coverage with isolated thunderstorms. Based on
latest runs of CAMs, convection should dissipate shortly after
sunset. I will update the forecast to adjust PoP and sky through
the late evening hours.

Previous Discussion:
This Evening and Tonight: Sfc high pressure will remain
centered north of the area while a mid/upper lvl trough of low
pressure lingers over the Southeast United States. Abundant sfc
heating along with onshore winds driving PWATs around 1.5 inches
over land will continue to fuel late afternoon/early evening
convection over parts of the area, mainly along a seabreeze
circulation currently near the Southeast South Carolina coast
and well inland where an ongoing cluster of thunderstorms
remains just out of the forecast area north of I-16. Weak shear,
weak lapse rates and lower SBCAPE and DCAPE than the previous
day suggest sub-severe thunderstorm activity in the area.
However, a few strong, but sub-severe wind gusts are possible
with thunderstorms before sunset. Showers/thunderstorms could
linger into early overnight hours, but the bulk of precip
activity will likely diminish within an hour or two of sunset
due to a loss of daytime heating. Diminishing cloud cover should
support decoupling winds and another fair radiational cooling
night. Guidance suggests temps will dip into the mid/upper 60s
inland to lower 70s near the coast. These temps along with
dewpts remaining in the mid/upper 60s inland to around 70 closer
to the coast suggests the potential of fog developing over some
locations after midnight, especially where showers and/or
thunderstorms are experienced this evening.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH MONDAY/...
Large scale ridging will be in place over the eastern third of the
country while Maria passes well offshore. Surface high pressure will
be located to our north, generating mainly NE flow. The end will
result will be fairly tranquil weather for our area. Models indicate
some showers and/or thunderstorms across the southern half of our
area Saturday. But we`re mainly keeping POPs in the slight chance
category because moisture and instability is not impressive. The
northern half should be dry. Sunday and Monday are expected to be
completely dry for the area as the high becomes more dominant.
Despite NE flow, thicknesses support above normal temperatures with
highs in the mid to upper 80s and lows in the mid to upper 60s,
except around 70 degrees at the beaches.

&&

.LONG TERM /MONDAY NIGHT THROUGH FRIDAY/...
Inland high pressure will persist through early next week while
Hurricane Maria is forecast to remain offshore of the Southeast U.S.
coast as it tracks north through the middle of next week. While some
showers could skirt coastal areas early next week as Maria passes
by, persistent, weak NVA under high pressure should help
mitigate precipitation chances. For much of the week, mainly dry
conditions are anticipated with temperatures likely remaining
above normal.

&&

.AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
Evening convection dissipated just prior to the 0Z TAFs. Radar
rainfall estimates indicate that rainfall did occur within a few
miles of KCHS and KSAV, but the terminals remained dry. NAM12
sfc condensation pressure deficits keep values of 5 mbs or less
just inland of the coastal counties. I will use a TEMPO to
highlight MVFR fog between 8Z to 12Z. Light winds and fair
weather Cu should persists through the daylight hours.

Extended Aviation Outlook: Mainly VFR conditions expected. Low
probability for brief flight restrictions in early morning fog
and/or low clouds Saturday morning at either terminal. Low
probability for brief restrictions due to isolated shower and
thunderstorm Saturday afternoon/evening, primarily affecting
KSAV.

&&

.MARINE...
This Evening and Tonight: High pressure will prevail over the
coastal waters, but a fair pressure gradient will persist through
tonight while Hurricane Maria tracks north well off the Southeast
coast. In general, east/northeast winds will be no higher than
10-15 kt. Long period swells from Maria will also increase over
all waters through tonight, supporting a Small Craft Advisory
for hazardous seas in offshore Georgia waters through the night
and in northern South Carolina waters late tonight. Seas should
peak near 5-7 ft in offshore Georgia waters and 4-6 ft in northern
South Carolina waters late tonight. Elsewhere, seas ranging between
2-3 ft will build up to 3-5 ft after midnight.

Saturday through Wednesday: Swell from distant Hurricane Maria will
continue propagating through our waters during this time period.
Models indicate the highest wave heights should take place Sunday,
especially across the offshore GA waters. Small Craft Advisories
will remain in place most of this time period, gradually expiring by
the middle of next week as seas subside. Otherwise, winds are not
expected to be an issue.

Rip Currents: Swell from offshore Hurricane Maria will slowly build
at the beaches. This results in a moderate risk of rip currents
through Saturday. An elevated risk of rip current will persist
through early to mid next week.

&&

.TIDES/COASTAL FLOODING...
Powerful surf created by offshore Hurricane Maria will drive
significant wave run-up, which could cause further beach erosion
this weekend into early next week. High Surf Advisories may be
required. Also, the potential for shallow salt water flooding
will persist through early next week around the times of high
tide, particularly along the South Carolina coast.

&&

.CHS WATCHES/WARNINGS/ADVISORIES...
GA...None.
SC...None.
MARINE...Small Craft Advisory for hazardous seas from 11 AM Saturday to
     8 PM EDT Tuesday for AMZ352.
     Small Craft Advisory for hazardous seas until 4 PM EDT
     Wednesday for AMZ374.
     Small Craft Advisory for hazardous seas from 6 AM Saturday to
     4 PM EDT Wednesday for AMZ350.
     Small Craft Advisory for hazardous seas from 11 AM Saturday to
     5 PM EDT Monday for AMZ354.

&&

$$
NEAR TERM...NED
SHORT TERM...MS
LONG TERM...JMC
AVIATION...MS/NED
MARINE...DPB/MS
TIDES/COASTAL FLOODING...


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